Proton-selective coating enables fast-kinetics high-mass-loading cathodes for sustainable zinc batteries

The pressing demand for sustainable energy storage solutions has spurred the burgeoning development of aqueous zinc batteries. However, kinetics-sluggish Zn2+ as the dominant charge carriers in cathodes leads to suboptimal charge-storage capacity and durability of aqueous zinc batteries. Here, we discover that an ultrathin two-dimensional polyimine membrane, featured by dual ion-transport nanochannels and rich proton-conduction groups, facilitates rapid and selective proton passing. Subsequently, a distinctive electrochemistry transition shifting from sluggish Zn2+-dominated to fast-kinetics H+-dominated Faradic reactions is achieved for high-mass-loading cathodes by using the polyimine membrane as an interfacial coating. Notably, the NaV3O8·1.5H2O cathode (10 mg cm−2) with this interfacial coating exhibits an ultrahigh areal capacity of 4.5 mAh cm−2 and a state-of-the-art energy density of 33.8 Wh m−2, along with apparently enhanced cycling stability. Additionally, we showcase the applicability of the interfacial proton-selective coating to different cathodes and aqueous electrolytes, validating its universality for developing reliable aqueous batteries.

1. Beyond addressing the cathode challenges in aqueous zinc batteries, it would be insightful to explore the potential application of this strategy to improve the stability of the zinc anode, particularly concerning issues such as stripping/plating and dendrite formation.Is it possible to extend this strategy for Zinc anode? 2. Given the ultrathin nature of the 2DPM and its linkage through imine bonds, concerns about stability during repeated electrochemical processes have been raised.It is essential to include experimental data assessing the electrochemical stability of the individual 2DPM to provide a comprehensive evaluation of its performance over time.3. Clarification is sought regarding the choice of a two NVO/2DPM cathode configuration in the pouch cell demonstration, as illustrated in Supplementary Fig. 27.Could you please give some explanation about this specific design?4. To ensure the work is up-to-date and relevant in the context of recent advancements in the field, it is recommended to cite recent important publications, such as Nat.Commun.2022, 13, 5348; Nat.Commun.2023, 14, 6526 (2023); Angew.Chem.2023, 135, e202307. 5. To address confusion regarding the calculation of the H+/Zn2+ insertion ratio, it would be better to provide a thorough explanation of the calculation methodology.

Reviewer #2 (Remarks to the Author):
Guo et al. present an innovative approach involving a 2D polyimine nanomembrane-based proton-selective coating of Zn battery cathodes to achieve fast kinetics and high mass loading.The artificial 2DPM coating with well-designed structures demonstrates a notable H+ flux and an excellent H+/Zn2+ transport selectivity, transitioning the cathode mechanism from sluggish Zn2+-dominated to fast-kinetics H+-dominated intercalation electrochemistry.Consequently, standard NVO cathode combined with 2DPM exhibits a theoretically close capacity, exceptionally high areal capacity, state-of-the-art energy density, and improved cycling stability.Also, the universal effect of such a coating strategy is also validated by assessing two other cathode materials.Overall, the proton-selective coating strategy for Zn battery cathodes is new and appears highly effective.Most statements and conclusions of this work are convincing with solid experimental support.Therefore, I find this study novel and impactful enough for the publication in Nat.Commun.However, my below concerns should be addressed before acceptance.1.In addition to achieving high ion transport flux and excellent ion selectivity, an ideal interfacial coating should also possess poor electron conductivity.The authors should provide information and related discussion on the electron conductivity of 2DPM? 2. Structural stability of 2DPM is surely a critical consideration during repeated charge and discharge processes.The authors should present the electrochemical stability result of 2DPM after cycling? 3.For practical applications, a high mass loading of active material is necessary, ideally exceeding 10 mg cm−2.Did the authors test the specific capacity variation of the NaV3O8•1.5H2Ocathode at different mass loadings?It is appreciated to show the relevant result.4. In DFT calculations, it is crucial to consider water molecules, as ions transport in a water environment.Did the authors simulate the water conditions in the DFT calculations?The related discussion should be added.5. Minor issue: the authors are suggested to keep the consistency in abbreviations throughout the manuscript, such as NVO with 80 nm 2DPM.There is no need to re-denote 2DPM-80-covered NVO in the boosted charge-storage kinetics section.

Reviewer #3 (Remarks to the Author):
The authors describe in the manuscript an ultrathin 2D polyimine membrane (2DPM) having dual ion-transport nanosized channels and densely distributed proton conductive groups for aqueous zinc batteries.They demonstrate high proton flux and superior proton/zinc cation transport selectivity in permeation tests of the titled 2DPM.The unique features of the membrane are proposed to lead to fast kinetics of proton-dominated Faradic reactions within high mass loading of sodium vanadium oxide cathodes.Besides, high areal capacity and energy density as well as remarkable cycling stability are reported.Furthermore, the concept of interfacial proton-selective transport is demonstrated to be applicable to other cathodes and aqueous electrolytes.Overall, this is a piece of nice work reporting a novel membrane coating to regulate ionic transport for aqueous multivalent metal ion batteries.I'd like to recommend publication of this study after further considering the following points.1, It should be noted that the concentration of proton in a 2M ZnSO4 aqueous solution is very limited as compared to Zn cations.In this case, can proton act as a suitable charge carrier?When proton permeation is much faster than Zn cations, the concentration of proton will be in an unbalanced state, and thus cause drastic change of the local environment of the counter electrode (i.e., zinc anode).2, What is the pH value and pH value change in the electrolyte during the discharge and charge of the cathode?3, What about the effect of the membrane thickness on the ionic diffusivity/selectivity and thus the electrochemical performance?4, The coating of ionic screening layer on cathode may impede electronic conduction.Any investigation along this line?5, Is the superior electrode kinetics partially ascribed to the 1D features of the vanadate cathode materials?

To Reviewer 1:
In this study, the researchers have detailed the development of an ultra-thin 2D polyimine nanomembrane (2DPM) featuring innovative dual ion-transport nanochannels and densely distributed proton-conduction groups.This configuration facilitates a heightened ion flux and exceptional H + /Zn 2+ selectivity at the cathode/electrolyte interface within aqueous zinc batteries.The thickness of the 2DPM allows for precise regulation of the H + and Zn 2+ charge carrier ratio, effectively addressing the challenge of Zn 2+ insertion in the cathode and realizing theoretical-close electrochemical performance.The introduction of the 2DPM presents an intriguing and profound strategy for achieving controllable charge carrier regulation in zinc batteries, leveraging the atomic-precise nature of the membrane to greatly enhance charge-storage capacity and durability.This research holds significant scientific merit in the context of battery studies, offering valuable insights for the design of high-performance batteries with a universal artificial cathode/electrolyte interface.However, for a more comprehensive understanding and to address potential concerns, I would recommend some minor revisions before acceptance by Nature Communications.Additionally, several questions have been raised: Response: We appreciate the positive comment of the reviewer.Additional experiments and revisions have been conducted according to your following comments.

1.
Beyond addressing the cathode challenges in aqueous zinc batteries, it would be insightful to explore the potential application of this strategy to improve the stability of the zinc anode, particularly concerning issues such as stripping/plating and dendrite formation.Is it possible to extend this strategy for zinc anode?
Response: Thank you for the insightful comment.In principle, the interfacial coating of 2D polymer membrane could be a promising strategy to address the challenges of Zn metal anodes.Ideally, such a coating would function akin to a solid-electrolyte interphase, isolating the Zn metal from direct contact with the electrolyte.This preventive action would mitigate parasitic Zn corrosion and inhibit the hydrogen evolution reaction.Meanwhile, the coating should selectively transport Zn 2+ , promoting a rapid and homogenous Zn 2+ flux on the Zn metal surface.Such a strategy would guide uniform and nondendritic Zn deposition.In this sense, high Zn 2+ conductivity/selectivity and hydrophobicity will be pursued criteria for the 2D polymer coating [1][2][3] , and the reported 2DPM in our study may not fully meet these criteria.Indeed, rationally designing and synthesizing 2D polymer membranes for Zn metal anodes remains an avenue for future exploration.We have added the corresponding discussion to the revised manuscript.

2.
Given the ultrathin nature of the 2DPM and its linkage through imine bonds, concerns about stability during repeated electrochemical processes have been raised.It is essential to include experimental data assessing the electrochemical stability of the individual 2DPM to provide a comprehensive evaluation of its performance over time.

3.
Clarification is sought regarding the choice of a two NVO/2DPM cathode configuration in the pouch cell demonstration, as illustrated in Supplementary Fig. 27.Could you please give some explanation about this specific design?
Response: We appreciate the valuable question.The pouch cell demonstration serves as a proof of concept to illustrate the practicality of our interfacial coating strategy for large-scale battery devices.
The chosen device configuration, utilizing two NVO/2DPM cathodes, was primarily employed to minimize the capacity gap between the anode and cathode in the device, aiming to achieve a suitable N/P ratio.We have clarified it in our revised manuscript.

4.
To ensure the work is up-to-date and relevant in the context of recent advancements in the field, it is recommended to cite recent important publications, such as Nat.Commun.2022, 13, 5348; Nat.
Response: Thank you for sharing these pioneering studies.We have included these references in the revised manuscript.

5.
To address confusion regarding the calculation of the H + /Zn 2+ insertion ratio, it would be better to provide a thorough explanation of the calculation methodology.
Response: We appreciate the valuable suggestion of the reviewer.The H + /Zn 2+ insertion ratio was determined following a method reported previously. 4Specifically, the total electron transfer number (n) per stoichiometric unit of NaV3O8•1.5H2Ocan be obtained using equation (R1), where C1 is the measured specific capacity, and C2 is the theoretical specific capacity of NaV3O8•1.5H2O.Meanwhile, the Zn/V atomic ratio (RZn/V) in the fully discharged electrode is quantified by the inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis.With n and RZn/V, the H/V atomic ratio (RH/V) in the fully discharged electrode can be estimated according to equation (R2).Finally, the H + /Zn 2+ insertion ratio (RH/Zn) can be derived based on equation (R3).The detailed explanation has been added into the revised manuscript.

3.
For practical applications, a high mass loading of active material is necessary, ideally exceeding 10 mg cm −2 .Did the authors test the specific capacity variation of the NaV3O8•1.5H2Ocathode at different mass loadings?It is appreciated to show the relevant result.

Figure R4a
compares their GCD curves at a current density of 0.1 A g −1 .Along with the mass loading increase from 2 to 30 mg cm −2 , the specific capacity slightly drops from 475.4 to 421.2 mAh g −1 .
Meanwhile, the areal capacity experiences a significant enhancement from 0.95 mAh cm −2 to an ultrahigh value of 12.64 mAh cm −2 (Figure R4b).This result further highlights the effective role of the 2DPM coating in boosting the charge-storage kinetics of high-mass-loading cathodes for aqueous Zn batteries.The corresponding discussion has been added to the revised manuscript.

4.
In DFT calculations, it is crucial to consider water molecules, as ions transport in a water environment.Did the authors simulate the water conditions in the DFT calculations?The related discussion should be added.

Response:
In fact, the effect of water environment was considered in our DFT calculations.Specifically, we introduced a dielectric constant parameter (78.4) to mimic the water environment.This approach aligns with established practices in similar simulation studies [5][6][7] .We have added this explanation to the revised manuscript.

5.
Minor issue: the authors are suggested to keep the consistency in abbreviations throughout the manuscript, such as NVO with 80 nm 2DPM.There is no need to re-denote 2DPM-80-covered NVO in the boosted charge-storage kinetics section.
Response: Thank you for the kind reminder.We have thoroughly checked all abbreviations and ensured that their definitions appear only once in the revised manuscript.

To Reviewer 3
The authors describe in the manuscript an ultrathin 2D polyimine membrane (2DPM) having dual iontransport nanosized channels and densely distributed proton conductive groups for aqueous zinc batteries.They demonstrate high proton flux and superior proton/zinc cation transport selectivity in permeation tests of the titled 2DPM.The unique features of the membrane are proposed to lead to fast kinetics of proton-dominated Faradic reactions within high mass loading of sodium vanadium oxide cathodes.Besides, high areal capacity and energy density as well as remarkable cycling stability are reported.Furthermore, the concept of interfacial proton-selective transport is demonstrated to be applicable to other cathodes and aqueous electrolytes.Overall, this is a piece of nice work reporting a novel membrane coating to regulate ionic transport for aqueous multivalent metal ion batteries.I'd like to recommend publication of this study after further considering the following points.
Response: We appreciate the positive comment of the reviewer.Additional experiments and discussions have been conducted to address the following concerns.

1.
It should be noted that the concentration of proton in a 2 M ZnSO4 aqueous solution is very limited as compared to Zn cations.In this case, can proton act as a suitable charge carrier?When proton permeation is much faster than Zn cations, the concentration of proton will be in an unbalanced state, and thus cause drastic change of the local environment of the counter electrode (i.e., zinc anode).

2.
What is the pH value and pH value change in the electrolyte during the discharge and charge of the cathode?
Response: Thank you for the constructive comments.Since Q1 and Q2 address similar concerns, we are combining our responses to these two comments.
Indeed, the initial concentration of H + in Zn salt electrolytes is limited (5 × 10 −5 mol L −1 for 2 M ZnSO4, pH = 4.3).However, it is essential to note that the water solvent in the electrolyte can act as a proton reservoir.The consumption of H + in the electrolyte would disrupt the equilibrium of the hydrolysis reaction of Zn 2+ (equation (R4)), triggering the generation of more H + charge carriers for the cathode.
According to your question (Q2), we further evaluated the electrolyte pH evolution during a discharge/charge cycle in a 2-electrode Swagelok cell.Due to the limited amount of the used electrolyte, we were not able to test the pH with a standard pH meter.Instead, pH paper strips were employed to assess the pH variation (Figure R5).As expected, the insertion of H + into the cathode causes a slight pH increase within a range of 4~6.
[Zn(H2O)6] 2+ +H2O ↔ [Zn(H2O)6-x(OH)] (2-x)+ + xH + (R4) We understand the reviewer's concern that the H + -involved cathode reaction could lead to changes in the electrolyte environment, accelerating the parasitic reactions of the Zn metal anode.Specifically, the involvement of H + charge carriers was identified as a crucial reason for the formation of the passivation layer (i.e., Zn4SO4(OH)6•4H2O) on the Zn anode 4,8,9 .However, considering the substantial benefits of H + charge carriers brought for the cathode, such as fast reaction kinetics, high mass loading, and large areal capacity, we believe that H + can be considered suitable charge carriers for cathodes.We certainly agree with the reviewer that particular attention should also be paid to protecting Zn metal anodes when full devices are assembled for practical applications.To this end, a range of previously reported strategies could be adopted, such as interphase construction and electrolyte additives with pH-adaptive capability [10][11][12][13][14][15] .All these discussions have been included in the revised manuscript.To evaluate the impact of membrane thickness on the electrochemical performance of NVO, we conducted GCD measurements at various current densities for the NVO electrodes coated with the 2DPM membrane.The calculated specific capacities of all electrodes are summarized in Figure R7a.
All the 2DPM membranes could boost the charge-storage capability of NVO, and the improvement degree of the specific capacity follows the trend of 2DPM-20 < 2DPM-60 < 2DPM-100 < 2DPM-80.
This trend is consistent with the H + /Zn 2+ selectivity trend illustrated in Figure R6c.Meanwhile, the quantity of H + charge carriers is estimated by considering the Zn/V atomic ratio and the total charge transfer per V atom. Figure R7b displays the charge carrier ratios (H + /Zn 2+ ) of all the electrodes.The contribution of H + charge carriers to the total charge storage of different electrodes matches well with the H + /Zn 2+ selectivity trend of the employed 2DPM.Specifically, 2DPM-80 empowers VNO with the largest H + /Zn 2+ ratio of 3.5, which contrasts with the pristine VNO with a low H + /Zn 2+ ratio of 0.4.The high H + /Zn 2+ ratio as the charge carriers exactly equal to the H + /Zn 2+ selectivity of 2DPM-80 in 2 M ZnSO4 as measured in Figure R6c.All these discussions can be found in the revised manuscript.

:
In fact, the excellent electrochemical stability of 2DPM in mild acid electrolyte was well demonstrated in our study.Specifically, NVO/2DPM after the cycling test was disassembled from the cell and subjected to scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) characterizations (Supplementary Fig.29).As revealed, 2DPM remains tightly covering the NVO surface with all characteristic FTIR peaks detected, verifying the robust electrochemical stability of 2DPM during repeated charge/discharge cycles.To further address your concern, we have conducted a galvanostatic charge/discharge (GCD) measurement of a 2DPM-80 electrode (2DPM-80-covered carbon paper) in a 2-electrode Swagelok cell, employing Zn foil as the counter electrode and 2 M ZnSO4 as the electrolyte.As depicted in Figure R1, 2DPM exhibits nearly identical GCD profiles, further verifying its excellent electrochemical stability within the potential range of 0.3~1.5 V vs. Zn/Zn 2+ .The corresponding discussion has been added into the revised manuscript.
Guo et al. present an innovative approach involving a 2D polyimine nanomembrane-based protonselective coating of Zn battery cathodes to achieve fast kinetics and high mass loading.The artificial 2DPM coating with well-designed structures demonstrates a notable H + flux and an excellent H + /Zn 2+ transport selectivity, transitioning the cathode mechanism from sluggish Zn 2+ -dominated to fast-kinetics H + -dominated intercalation electrochemistry.Consequently, standard NVO cathode combined with 2DPM exhibits a theoretically close capacity, exceptionally high areal capacity, state-of-the-art energy density, and improved cycling stability.Also, the universal effect of such a coating strategy is also validated by assessing two other cathode materials.Overall, the proton-selective coating strategy for Zn battery cathodes is new and appears highly effective.Most statements and conclusions of this work are convincing with solid experimental support.Therefore, I find this study novel and impactful enough for the publication in Nat.Commun.However, my below concerns should be addressed before acceptance.Response:We appreciate the positive comment of the reviewer.Additional experiments and discussions have been conducted to address the following concerns.

2 .
Structural stability of 2DPM is surely a critical consideration during repeated charge and discharge processes.The authors should present the electrochemical stability result of 2DPM after cycling?Response: We appreciate the constructive suggestion of the reviewer.In fact, the excellent electrochemical stability of 2DPM was well demonstrated in our study.Specifically, NVO/2DPM after the cycling test was disassembled from the cell and subjected to scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) characterizations (Supplementary Fig.29).As revealed, 2DPM remains tightly covering the NVO surface with all characteristic FTIR peaks detected, verifying the robust electrochemical stability of 2DPM during repeated charge/discharge cycles.To further address your concern, we have conducted a galvanostatic charge/discharge (GCD) measurement of a 2DPM-80 electrode (2DPM-80-covered carbon paper) in a 2-electrode Swagelok cell, employing Zn foil as the counter electrode and 2 M ZnSO4 as the electrolyte.As depicted in FigureR3, 2DPM exhibits nearly identical GCD profiles, further verifying its excellent electrochemical stability within the potential range of 0.3~1.5 V vs. Zn/Zn 2+ .The corresponding discussion has also been added to the revised manuscript.

Figure
Figure R4. a GCD curves of NVO/2DPM electrodes with different mass loadings.b Corresponding areal capacity variation as a function of mass loading.

Figure R5 . 3 . 2 h − 1
Figure R5.The pH evolution in the electrolyte during one discharge/charge cycle of the NVO/2DPM electrode.

Figure R7. a Figure R8 .
Figure R7. a Specific capacities at various current densities and b the charge carrier ratios (H + /Zn 2+ ) of NVO covered by 2DPM with different thicknesses.